A conventionally known fluid heating device is shown in the patent document 1 wherein a plurality of rounds of a helical conventional tube which form a secondary coil are electrically connected by the use of an electrical connecting member that extends in an axial direction of the helical shape by means of welding or the like so as to constitute a short circuit so that electrical reactance is reduced and heating efficiency is improved.
If induction heat or electrical heat is applied to the helical conductor tube, a larger electric current flows in an inner circumference side of the conductor tube with a shorter length and a lower electrical resistivity than in an outer circumference side thereof, and a temperature in the inner circumference side becomes higher than that in the outer circumference side. As a result of this, thermal expansion becomes greater in the inner circumference side of the conductor tube than in the outer circumference side so that a mutual disposition of the conductor tube changes in a direction of unwinding the helical shape of the conductor tube.
In addition, since a temperature of an exit side of the fluid becomes higher than that of an entrance side, a diameter in the exit side becomes bigger so that a shape of the helical conductor tube viewed from the front changes into a trapezoid shape.
However, if the electrical connecting member is fixed to required positions of the conductor tube similar to in the conventional fluid heating device, considerable stress is generated when the conductor tube is about to change its shape and may cause an accumulation of fatigue or breakage of the conductor tube or the electrical connecting member or at a portion where the conductor tube is fixed to the electrical connecting member.